Mercury is flowing with the melt. A new study from Sweden finds that thawing permafrost in a northern peat bog is releasing the toxic metal into a nearby lake. That pattern could become widespread as global temperatures rise, the authors warn.

Researchers have long known that the vast peatlands that ring the northern hemisphere are packed with mercury, which can be deadly to life. Some came from natural sources; the rest drifted in from industrial-age burning of fossil fuels such as coal. For centuries, much of the accumulated mercury has been locked in permafrost, the year-round frozen layer that lies just beneath the surface in many northern climes. Recently, however, scientists have begun to speculate about what might happen to this trove as global warming takes hold.

A hint comes from a research team led by Johan Rydberg of Umea University in Sweden. They studied how mercury levels had changed over the last 1,000 years at the Stordalen mire in northern Sweden, and a nearby lake called Inre Harrsjon. Using core samples taken from the bog and lake-bottom sediments, they were able to document shifting mercury concentrations in each environment, and then compare the trends to past climate data. In general, they found that mercury spikes in the lake sediments corresponded to warming periods – including an uptick linked to warmer winter temperatures over the last few decades. Ironically, the recent rise in lake levels of mercury came even as less mercury pollution was reaching the peat, perhaps in part due to increased pollution controls. Overall, they found that sediment mercury levels are now rising at 8.3 micrograms per square meter per year, a rate not seen in several centuries, they report in the current issue of Science of the Total Environment.

The findings suggest “that there is a very real potential that a substantial amount of mercury, and other organically bound and stored contaminants, might be released into arctic and sub-arctic surface waters from thawing permafrost,” the authors write. But not all peat is created equal, they note, so researchers will have to study bogs in other regions to know exactly how they respond to warmer temperatures. – David Malakoff